CCD Readout-Mode Selection Guide

Important：

CCD was changed from SITe chip to MIT chip in September 2001. There are the following issues.

Changes:

Readout time, readout noise, sensitivity, chip alignment, overscan region

No changes:

pixel scale, CCD dimension

Imaging:

• Normally, 1x1 binning is used for scientific exposures.
• The particular mode used with SITe (ultrafast mode) is not available with MIT.

Spectroscopy:

• S/N improvement is done by evaluating the slit width and the seeing size when executing binning.
• The binning size in spatial direction is estimated by 1/2~1/3 of the seeing size.
• The binning size in slit direction is estimated by 1/2 of the slit size.
• Partial readout mode is used for target acquisition, and it is necessary to take at least three exposures for the images (slit image, object image, image on the slit).

Notes

Overscan:

• Overscan region is installed in every frame in order to dump the constant viruses.
• Overscan X size = 58 (For 1x1 bin）.
• x=1~(overscan size) and Y=* are the overscan for chip1.
• x=(image X size)-(Overscan size) ~ (image X size) and Y=* are the overscan for chip 2.
  (Note that the overscan appears from different directions for chip1 and chip2.)

  |-----|| ||-----|  Y
  |     ||G||     |  ^
  |Chip ||A||Chip |  |
  |  2  ||P||  1  |  |
  |     || ||     |  |
  |-----|| ||-----|  |
        OS OS
       ------> X
  OS: Overscan
  

• The gap size between chips 1 and 2 is approximately 5 arcsec (49 pix) (Barely narrowed down from that in our previous SITe chips).This measures the distance from the last sensitive pixel in X on chip2 to the first sensitive pixel on chip1. In specific, it seems that effective pixels are lost due to the overscan installed on the gap between the chips, but there is no effective pixels lost since the overscan is read from the empty current with no electric charge flowing.

Actual read-out time:

• CCD should be wiped for 6 seconds before every exposure. It will be a dead time when taking continuous scientific exposures, but in a lot of cases when the CCD wiping takes place at the same time as the slit is moving, no time is lost.
• The following readout time is applied in each of the modes. The table indicates the actual time including the time spent for the data transfer and the FITS transfer added to the time spent for reading out the CCD. There is a slight difference in the readout time according to the computer loads, etc.
• With the change to MIT:
  • There is no longer the ultrafast readout mode; The normal readout mode in MIT is about as fast as the fast readout mode in SITe.
  • The readout mode in MIT is shown to be 2-3 times as fast as that in SITe.